If you're in and around Tokyo, there are cosplay events happening every single weekend at places like Tokyo Big Sight, TFT, Heisei Plaza, Tokyo Leisure Land, Sanrio, Toshimaen, Tokyo Dome City, etc. Not to mention, dozens of studios intended for cosplayers and their photographers are open every day of the week. Most of the events taking place don't get a lot of hype because they are for cosplay only (no selling booths, stages, or panels), but if the cosplay is all you're interested, then there are hundreds of events to choose from. The trick is knowing enough Japanese to be able to navigate the event schedules and understand the rule constraints.

If you want to try looking things up on your own, this is a good resource: http://ja.curecos.com/event/
1. Select a date from the calendar.
2. Select 東京都 from the drop down menu, if you want to limit your search to Tokyo.
3. Press the white button marked 検索.
That will give you a list of all of the cosplay events and open cosplay studios around Tokyo and Odaiba for any given day.